The Internet protocol (IP) multimedia subsystem (IMS) defined by the 3rd generation partnership project (3GPP) represents a PS service delivery platform architecture for the provision of IP multimedia services within emerging all-IP network environments. The IMS comprises three main components: the serving call session control function (S-CSCF) on a control layer, and the home subscriber server (HSS) as well as the session initiation protocol (SIP) application server (SIP-AS) on an application layer.
During operation, the S-CSCF forwards SIP messages received from user terminals or other network components to the SIP-AS. Based on the received SIP messages, the SIP-AS determines the services that are to be provided to a particular user. During the execution of a particular service, the SIP-AS may require additional information about a user and may to this end communicate with the HSS. The HSS stores a service profile for each user and can thus be regarded as the equivalent to the home location register (HLR) in conventional second (and third) generation networks such as GSM (global system for mobile communication) networks.
IMS has been preceded by a change in network topology—that is, the migration towards layered network architectures. Whereas in conventional second generation networks a single component, the mobile switching center (MSC), handles both call control and connectivity, in layered networks these functionalities have been split. More specifically, call control is handled by MSC servers (MSC-S) in the control layer, whereas connectivity is handled by media gateways (MGW) in the connectivity layer. This separation of call control and connectivity is also referred to as mobile softswitching (MSS).
The MSC-S handles the network signalling and intelligence for setting up, releasing and monitoring CS connections. The MGW, on the other hand, processes and manages the transport of CS payload traffic (such as voice or data traffic). The MGW additionally provides interconnections to external networks including public switched telephone networks (PSTNs) and public land mobile networks (PLMNs).
Network operators with an installed MSS environment or with the intention to deploy MSS will want to ensure a smooth transition from MSS and the CS domain to an all-IP solution. From a migration perspective, and also with regard to a reuse of installed equipment, operators will preferably use CS services (such as CS telephony services) during a co-existence period also for IMS subscribers having PS network access.
The object underlying the invention is to propose a technique for interconnecting CS and PS domains. In particular, a technique for providing CS services to a network component having a PS network access is required.